Antipsychotics are administered to children and adolescents for a number of disorders with chronic use often continuing into adulthood. Yet, little is known about short- and long-term effects of these agents on the developing brain and behavior. Research on the effects of atypical antipsychotics (e.g., clozapine) that do not produce extrapyramidal motor effects is particularly lacking. The major hypotheses of this grant proposal are that (1) developing animals are more sensitive to the effects of dopamine antagonists, including antipsychotics, on motor processes than are adult animals and (2) chronic dosing with antipsychotics during development produces long-term changes in response to challenges with dopaminergic agents in later life such that animals are more sensitive to the effects of dopamine agonists and less sensitive to those of dopamine antagonists. In order to test the first hypothesis, rats of different ages (postnatal day 22 to adult) will be administered acute doses of selected antipsychotics; subsequently, they will be evaluated in behavioral procedures designed to measure motor activity (locomotion and catalepsy). In order to test the second hypothesis, rats will be chronically injected with selected antipsychotics during development. After reaching adulthood, these rats will be evaluated in behavioral procedures to evaluate motor activity (locomotion and catalepsy), cognition (sensorimotor gating, acquisition of a response, short-term memory), and the reinforcing efficacy of food. In addition to baseline activity in these procedures, the effects of challenges with antipsychotics and dopamine agonists will also be assessed in these rats. In order to determine possible underlying changes in dopamine receptor binding and distribution, autoradiography of the brains of rats that received identical chronic injection regimens will be performed using radioligands selective for dopamine D1 and D2 receptors. The proposed studies will provide empirical information on acute and long-term effects of traditional and atypical antipsychotics on the developing brain and behavior. This information will help to provide a more rational basis for making treatment decisions concerning children and adolescents who may benefit from treatment with an antipsychotic. [unreadable] [unreadable]